1. Field of the Invention
This invention relates to a control apparatus for use in a power transmission system of construction vehicles, and more particularly to a control apparatus for a power transmission of construction vehicles such as, for example, a wheel loader or the like, the vehicles having a power transmission system in which a torque converter with a hydraulically actuatable or engageable stator clutch and a transmission gear adapted to effect speed change by means of a hydraulically engageable and disengageable clutch are equipped, and further having at least a work implement such as a hydraulically actuatable bucket or the like.
2. Description of the Prior Art
In the conventional power transmission system for construction vehicles of the kind specified, an actuating unit for a stator clutch of a torque converter and a speed change clutch of a transmission gear are connected through directional control valves or selector valves respectively installed in their hydraulic circuits with a hydraulic fluid supply source, and the arrangement is made such that the power transmission gear is controlled by changing over each selector valve properly to change the running speed of the vehicle.
In the above-mentioned conventional construction vehicle such as, for example, a wheel loader or the like, acceleration of the engine is normally made by the operator depending on the load applied to the bucket during excavation so as not to cause slipping of tire-wheels; however, in many case, this operation requires skilled technique for excavating different objects.
Stating in brief, in case excavation work is carried out by a wheel loader having a fixed displacement torque converter, if the bucket is subjected to a sufficient load from the object being excavated during the excavation work, slip of tire-wheels does not occur even when the engine is run under fully open throttle condition, whilst the load is lower than a predetermined value, slip of the tire-wheels occurs thus causing wear of them.
Further, some of such wheel loaders equipped with a variable displacement type torque converter for the purpose of preventing slip of tire-wheels. However, this arrangement is incapable of rendering the displacement variable in accordance with the load imposed on the bucket, and if the displacement is set at a lower value to prevent slip of tire-wheels, the tire-wheel driving force becomes insufficient under the maximum bucket loading condition.
FIGS. 1 to 3 are views for explaining the above-mentioned problem, wherein W denotes the vehicle weight; W.sub.F the load on front wheels; W.sub.R the load on rear tire-wheels; F the load on the bucket; .mu. the frictional coefficient of the road surface, and .mu. (W.sub.F +W.sub.R) which is the critical tangent force for tire-wheel slip.
FIG. 2 shows the relationship between the load applied to the bucket, the critical tangent force for tire-wheel slip and the tire-wheel driving force. In this drawing, reference character "X" denotes the tire-wheel driving force obtained when a torque converter of fixed displacement type is used. In this case, when the bucket is subjected to a low load, slip of tire-wheels occurs in the zone designated by an oblique line thus causing ineffective power consumption and accelerated wear of tire-wheels. Further, under the maximum bucket loading condition, the tire-wheel driving force reaches a point C.sub.1 causing a shortage in driving force.
Whilst, if a torque converter of variable displacement type is used and set at an original point "a" for critical tangent force for tire-wheel slip so as not to cause slip of tire-wheels, then the tire-wheel driving force diagram shown by reference character Y is obtained. In this case too, under the maximum bucket loading condition, the tire-wheel driving force reaches the point "c" causing a shortage in driving force.
FIG. 3 is a diagram showing the relationship between the engine's rotating speed and the torque applied to the engine output shaft.
In order to alleviate the above-mentioned problems, it is required for the operator to make adjustment by means of the accelerator so as not to cause slip of tire-wheels.
The U.S. Pat. No. 4,004,417 discloses, as one of measures to solve the above-mentioned problems in the conventional construction vehicles, provision of a slipping clutch between the output shaft of the engine and the impeller of the torque converter to accurately transmit a driving force required by the tire-wheels upon transmission of the driving force produced by the engine through the torque converter, the slipping clutch being arranged to be controlled precisely. However, the aforementioned problems cannot be effectively solved only by the control of the slipping clutch, since the conditions such as the vehicle speed and loading applied to the work implement need to be taken into consideration at the same time.